Sum if

Attached is a file where I have sales listed out by month for 2013 and 2014. In N2, I would like a formula telling it to sum only the months where there is a total for those months in 2014. So, it would currently add Oct thru Aug, leaving off Sept 2013 because there is no total entered for Sept 2014. Plus, I would like M4 for be blank because there is no total entered in Sept 2014. Thank for your help.